Had it happened in this country, the two knife-wielding friends of the dead mugger could very well be charged with a capital crime in the death of their dead mugger friend.
Felony Murder statutes in many of our States are written broadly enough to hold accomplices liable even for the death of a fellow perp - not just the death of an innocent victim.
I find it very difficult to sympathise with felons who wind up dead during the commission of their crimes. Your mileage may vary.
Maybe things are different in The Netherlands, but in most places here, pulling a gun on someone in close quarters will be found by police, prosecutors and courts as sufficient justification for any self-defense response by the intended victim(s), even if it ends in death for the criminal.
Pretty easy for the dead mugger to avoid, as it's pretty unlikely the elderly ex-Marine went looking for someone to kill that day. The mugger and his partners in crime, however, were out to commit some felonies. And thankfully, even in Costa Rica, the authorities exhibit sensible and reasonable responses when unarmed foreign tourists are ambushed by three armed thugs and things go wrong for one of the thieves.